1. Field of the Invention
This invention relates to a chroma encoder, and more specifically to a circuit which forms a chroma signal from color-difference signals.
2. Related Background Art
In a known digital circuit for forming either an NTSC standard or PAL standard chroma signal from color-difference signals, the band of each color-difference signal is limited by a low-pass filter. Then, the color-difference signal is clamped by a clamping circuit in such a way that a pedestal level of an A/D (analog to digital) converted color-difference signal (e.g., 8 bits in this case) corresponds to a predetermined "128" (2.sup.7) level, and the clamped color-difference signal is digitized by an A/D converter. Various digital processing steps are performed on the digitized color-difference signal, such as, converting the signal by a complement of two, adding a burst flag to the signal and performing time-division multiplexing on the signal to obtain a digital chroma signal. The digital chroma signal is then converted into an analog signal by means of a D/A (digital to analog) converter.
However, the above described circuit requires the correct adjustment of a clamp level of the clamping circuit, which is installed before the A/D converter, to a predetermined level because the clamp level affects the chroma signal as a carrier leak when the clamp level is beyond a predetermined value. The adjustment of the clamp level is not only difficult but also there is a possibility of causing a deviation of the clamp level by a change of temperature even if the adjustment is correct.